Method for operating a vacuum melting system, and vacuum melting system operated according to said method

ABSTRACT

A method operates a vacuum melting system for metallurgically treating molten steel. A vacuum melting system is operated according to the method. The acoustic signals generated in a ladle which receives the molten steel are detected by at least one structure-borne noise detector which is directly or indirectly acoustically coupled to the ladle, and the acoustic signals are used to ascertain the height or the thickness of the foamed slag which can be found in the ladle over the molten bath of the molten steel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to International Application No. PCT/EP2013/055949 filed on Mar. 21, 2013 and European Application No. 12 163 717.7 filed on Apr. 11, 2012, the contents of which are hereby incorporated by reference.

BACKGROUND

The invention relates to a method for operating a vacuum melting system. The invention also relates to a vacuum melting system operated according to said method.

In a vacuum melting system a steel melt, produced for example in a preceding step in an electric arc furnace, is subjected to post processing in order to eliminate undesired accompanying elements still contained in the steel melt. These types of vacuum melting systems are referred to as VD or VOD (Vacuum Decarburization or Vacuum Oxygen Decarburization) systems, depending on whether exclusively an inert gas or additional oxygen is used as the process gas. The process duration, i.e. the period of time until the desired content of disruptive accompanying elements is reached, substantially depends on the rate with which the process gases are blown into the steel melt. A feeding rate which is too low can lead to the temperature of the steel melts falling so far before the desired content of disruptive accompanying elements is reached that reheating of the steel melt in the pan or a complete post processing of the melt is required. A high feeding rate however, especially with vacuum oxygen decarburization, can lead to an overcooking or overfoaming of the melt, which is associated with a considerable and time-consuming subsequent cleaning effort.

The rate with which the process gas is fed to the steel melt is set manually in such cases by the operator observing the surface image of the melt and thus the height of the foamed slag in the pan by a camera and controlling the feeding rate accordingly. The process control is accordingly dependent on the experience and the attentiveness of an operator, so that incorrect or inefficient process controls cannot be avoided with any certainty. In addition incorrect operating states, such as can be caused for example by leakages of the vacuum system, will only be recognized with great difficulty or recognized very late.

SUMMARY

One possible object is therefore to specify a method for operating a vacuum melting system for metallurgical treatment of a steel melt, with which the process security is improved. Another potential object is to specify a vacuum melting system operated with this method.

The inventors propose a method in which acoustic signals created in the pan are picked up by at least one structure-born sound pick-up coupled acoustically indirectly or directly to the pan accommodating the steel melt and are employed for determining the height or the depth of the foamed slag located in the pan above the melt bed of the steel melt.

Through this measure the danger of an overcooking of the melt can also be recognized in good time and countermeasures, for example reduction or interruption of the feeding of process gas, can be initiated accordingly, which prevent overcooking.

The proposals are based in this case on the idea that acoustic signals arising in the pan during operation of the vacuum melting system, especially during blowing in of the process gas, depending on the point of origin of said signals and the propagation paths to a sound pick up associated therewith, exhibit characteristic properties which make it possible to derive from the acoustic signals information about the operating state of the vacuum melting system.

Height of the foamed slag is to be understood below as the location of the upper level of the foamed slag relative to a fixed reference point of the vacuum melting system. This can for example be the distance between the floor of the pan and the upper level. The height of the foamed slag is substantially determined in this case by its depth, since the height of the actual steel melt is practically constant.

Such overcooking or overfoaming can especially be safely prevented if the temporal differential quotient of the height or the depth of the foamed slag is determined. In this way a rapid rise in the height of the foamed slag is recognized in good time.

In an especially preferred embodiment the determined height of depth and/or its temporal differential quotient is employed for regulating the height of the foamed slag by controlling the feeding of a process gas into the pan. In this way the entire post-processing process executing in a vacuum system can be stabilized accordingly.

In a further advantageous form of embodiment the acoustic signals are also used for detection of a leakage in the vacuum melting system.

The inventors also propose a vacuum melting system that has at least one structure-born sound pick up acoustically coupled indirectly or directly to the pan for picking up the acoustic signals created in the pan and also a control and evaluation device with an algorithm implemented therein for determining the height or the depth and/or the temporal differential quotient of said height or depth of the foamed slag located in the pan above the melt bath from the acoustic signals picked up by the structure-born sound pick-up or pick-ups.

If the at least one structure-borne sound pick-up is fixed to the pan, the sound signals arising within the pan can be registered with high sensitivity.

The height or depth of the slag can be determined especially precisely if the at least one structure-borne sound pick-up is disposed in an upper area of the pan.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawing of which:

The FIGURE is a diagram of the vacuum melting system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

In accordance with the FIGURE a vacuum melting system comprises a vacuum vessel 2, which is sealed with a cover 4. Inserted into this vacuum vessel 2 is a pan 6 filled with a steel melt, to the underside of which a plurality of gas feed lines 8 for feeding process gas P1 are connected, of which only one is shown in the FIGURE for reasons of clarity. Vacuum vessel 2 and cover 4 accordingly form a system part surrounding the pan 6.

Shown as dashed lines is the form of embodiment of the so-called VOD vacuum melting system, in which oxygen can be introduced into the pan 6 as a further process gas P2 via a further gas feed line 10. In this form of embodiment the pan 6 is additionally covered by a protective cover 12 with which the ejection of slag by overfoaming can be reduced.

In the pan 6 there is a steel melt, which is composed of a liquid melt bath 14, of which the bath level 16 is located at a distance h from the floor of the pan 6, and a foamed slag 18 located above said bath, the depth of which is d, so that the upper level 20 is located at the height H=h+d above the floor of the pan 6.

Disposed on both the outer wall of the pan 6 and also on the wall of the vacuum vessel 2, as well as on the cover 4 of the vacuum vessel 2, are structure-borne sound pick-ups 30-1, 30-2, 30-3 and 30-4, with which the acoustic signals created within and in the vicinity of the pan 6, by a vacuum pump for example, are picked up.

The measurement signals MI, M2, M3 or M4 provided by the structure-borne sound pick-ups 30-1, 30-2, 30-3 and 30-4 in each case are forwarded to the control and evaluation device 40 in which they are evaluated and employed for determination of the height H or the depth d of the foamed slag 18.

The structure-borne sound pick-ups 30-3, 30-4, preferably permanently installed on the vacuum vessel 2 or on the cover 4, can also be disposed within the vacuum vessel 2. They are not coupled acoustically directly to the wall of the pan 6. Instead the acoustic signals created in the pan 6 are transmitted via corresponding structures to the wall of the vacuum vessel 2 or to the cover 4.

The structure-borne sound pick-ups 30-1, 30-2 disposed on the outer wall of the pan 6 and coupled acoustically directly to the wall of the pan the 6 are removable, i.e. fixed releasably to the pan 6 and are only coupled to the pan 6 with quick-release fastenings after insertion of the pan 6 into the vacuum vessel 2.

The sound arising in the pan 6 from blowing the process gas P1, P2 into the steel melt propagates within the melt bath 14 and within the foamed slag 18 outwards to the wall, wherein the foamed slag 18 has a sound deadening effect. In other words: The depth d of the foamed slag 18 and its height H or location within the pan 6 significantly influences the sound signal, especially picked up by the structure-borne sound pick-up 30-1 disposed in the upper area of the pan 6.

The created measurement signals MI, M2, M3 and M4 are subjected to signal analysis in the evaluation device 40 and the height of the foamed slag is determined with the assistance of a self-learning physical model. To do this the measurement signals MI, M2, M3 and M4 are subjected to a fast Fourier transformation for example. The frequency spectra generated in this way are compared with frequency spectra which have been measured in a preceding learning phase for different operating states of the vacuum melting system, especially at different pressure in the vacuum vessel 2, different feed rate of the process gases P1, P2 and also different heights of the foamed slag determined by recording with the camera. With the aid of learning and pattern detection algorithms, by comparing a real measured frequency spectrum with the frequency spectra obtained in the learning phase, the height H and especially the depth d of the foamed slag 18 or of its temporal differential quotient dH/dt or dd/dt respectively can then be determined, without observation with a camera being required for this.

Control signals S1 and S2, with which the feeding rate of the process gases P1, P2 is controlled, are created in the control and evaluation device 40 as a function of the determined height H or depth d and preferably of the determined differential quotients, to regulate the height of the foamed slag 18 to a constant value or at least to prevent an overfoaming of foamed slag 18.

In the exemplary embodiment shown a plurality of structure-born sound pick-ups are provided both on the pan 6 and also on the vacuum vessel 2. Basically however the method can also be performed with a single structure-borne sound pick-up 30-1, preferably disposed in the upper area of the pan.

By analysis of the acoustic signals the occurrence of operating states caused by leakages, for example a cover 4 not being correctly closed, can be recognized in good time and the corresponding deficiency accordingly rapidly rectified. Also in this case different operating states are set in the learning phase before the actual commissioning, for example operation of the vacuum system with correct and incorrectly closed cover, intentional setting of leakages, and the corresponding structure-born sound signals recorded. The frequency spectra of the measurement signals MI, M2, M3 and M4 obtained in this learning phase or stored as typical patterns, so that by comparing a frequency spectrum measured in real operation with the stored patterns, the occurrence and the cause, i.e. the location of a leakage can be established.

The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004). 

1-10. (canceled)
 11. A method for operating a vacuum melting system for metallurgical treatment of a steel melt, the steel melt being treated in a pan inserted in a vacuum vessel, the steel melt including a liquid melt bath and a foamed slag located on top of the liquid melt bath, the method comprising: directly and/or indirectly coupling at least one structure-borne sound pick-up to the pan; picking up, with the at least one structure-borne sound pick-up, acoustic signals generated within the pan; and determining at least one of a total height of the foamed slag and the liquid melt bath within the pan and a depth of the foamed slag above the liquid melt bath, based on the acoustic signals that are picked up by the at least one structure-borne sound pick-up.
 12. The method as claimed in claim 11, further comprising: determining at least one of a temporal differential quotient of the total height and a temporal differential quotient of the depth based on the acoustic signals that are picked up by the at least one structure-borne sound pick-up.
 13. The method as claimed in claim 12, further comprising: controlling a feeding of a process gas into the pan based on at least one of the total height and the depth and/or at least one of the temporal differential quotient of the total height and the temporal differential quotient of the depth.
 14. The method as claimed in claim 11, further comprising: detecting a leakage in the vacuum melting system based on the acoustic signals that are picked up by the at least one structure-borne sound pick-up.
 15. A vacuum melting system for metallurgical treatment of a steel melt, the system comprising: a vacuum vessel; a pan configured to be inserted inside the vacuum vessel and configured to hold the steel melt, the steel melt including a liquid melt bath and a foamed slag located on top of the liquid melt bath; at least one structure-borne sound pick-up indirectly or directly coupled to the pan and configured to pick up acoustic signals generated within the pan; and a control processor configured to determine at least one of a total height of the foamed slag and the liquid melt bath within the pan and a depth of the foamed slag above the liquid melt bath based on the acoustic signals picked up by the at least one structure-borne sound pick-up and/or at least one of a temporal differential quotient of the total height and a temporal differential quotient of the depth, based on the acoustic signals picked up by the at least one structure-borne sound pick-up.
 16. The vacuum melting system as claimed in claim 15, wherein the at least one structure-borne sound pick-up is fixed to the pan.
 17. The vacuum melting system as claimed in claim 16, wherein the at least one structure-borne sound pick-up is fixed to an upper area of the pan.
 18. The vacuum melting system as claimed in claim 15, wherein the vacuum vessel includes a cover configured to close the vacuum vessel and the at least one structure-borne sound pick-up is permanently installed on the vacuum vessel and/or the cover.
 19. The vacuum melting system as claimed in claim 15, wherein the control processor is further configured to regulate the total height or the depth of the foamed slag by controlling a feeding of a process gas into the pan as a function of at least one of the total height and the depth of the foamed slag and/or at least one of the temporal differential quotient of the total height and the temporal differential quotient of the depth of the foamed slag.
 20. The vacuum melting system as claimed in claim 15, wherein the control processor is further configured to detect a leakage in the vacuum melting system based on the acoustic signals picked up by the at least one structure-borne sound pick-up.
 21. The method as claimed in claim 11, further comprising fixing the at least one structure-borne sound pick-up to an upper area of the pan.
 22. The method as claimed in claim 11, further comprising permanently fixing the at least one structure-borne sound pick-up to the vacuum vessel and/or a cover of the vacuum vessel.
 23. The method as claimed in claim 11, further comprising fixing the at least one structure-borne sound pick-up to the pan after the pan is inserted into the vacuum vessel.
 24. The method as claimed in claim 11, further comprising determining the total height or the depth of the foamed slag by comparing the acoustic signals picked up by the at least one structure-borne sound pick-up with previously learned acoustic signals in a self-learning physical model.
 25. The method as claimed in claim 12, wherein the temporal differential quotient of the total height is represented as a change in the total height over time and the temporal differential quotient of the depth is represented as a change in the depth over time.
 26. The vacuum melting system as claimed in claim 15, wherein the vacuum vessel includes a cover configured to close the vacuum vessel.
 27. The vacuum melting system as claimed in claim 26, wherein the at least one structure-borne sound pick-up includes at least two structure-borne sound pick-ups fixed to the pan, at least one structure-borne sound pick-up fixed to the vacuum vessel, and at least one structure-borne sound pick-up fixed to the cover of the vacuum vessel.
 28. The vacuum melting system as claimed in claim 15, wherein the control processor determines the total height or the depth of the foamed slag and/or the temporal differential quotient of the total height or the temporal differential quotient of the depth of the foamed slag by comparing the acoustic signals picked up by the at least one structure-borne sound pick-up with previously learned acoustic signals in a self-learning physical model.
 29. The vacuum melting system as claimed in claim 15, wherein the temporal differential quotient of the total height is represented as a change in the total height over time and the temporal differential quotient of the depth is represented as a change in the depth over time. 